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I wonder if switching from memcached to redis would make a bottom line difference in terms of the number of instances needed (cost) and performance?

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I thought the same thing. I stuck with memcached for quite a while because I didn't see a real reason to switch to redis, but after hitting some repeated annoyances with memcached, I gave redis a try and very happy I did. It's much better. Is memcached even maintained anymore?

Yeah it absolutely is. In fact, there's been tons of large improvements as of late, with more coming.

Maybe a difference in performance, but caching comes down to bits-to-be-stored. That likely wouldn't change.

Perhaps they are already doing it, but couldn't they use client side compression as well?

redis does have some built-in features that automatically compact data. [0] I'm sure reddit has considered this, but worth noting.

[0] https://redis.io/topics/memory-optimization

client-side compression works out better for a couple of reasons:

1. You use less bandwidth in/out of the box. This matters in AWS because your bandwidth is (relatively) limited.

2. Your CPU usage to (de)compress is distributed over a larger fleet. The clients spend the CPU time, not the cache servers.

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